We assume that snakes kill with venom shot through their fangs, injection style. In fact, most snakes leak poison very slowly. Oddly this technique works quite well, and you can figure out why by considering the physics of ketchup in a bottle.

When a venomous snake bites, it has long fangs to bite far into its prey and push the venom deep into the wound. There the toxin incapacitates the prey and the snake enjoys a good meal. Using that model, most fangs should look like syringes. They should be long, hollow tubes through which venom is pushed. That isn't what researchers found when they started inspecting snake fangs. Only about one in seven venomous snakes had fangs through which they injected poison. Most had grooves in their teeth that poison simply flowed down. Since these snakes are still around, this predatory system has to work, but how exactly do the snakes ensure that the venom is pushed into the flesh of its prey?

One major reason why the grooved teeth work is the high surface tension of snake venom. The molecules in the venom hold on to each other tightly. Molecules along the surface of the venom form a kind of 'skin' that holds the body of liquid together. The suface tension exerts an inward pressure on the main body of liquid, so when the snake bites, and lets its poison drip down its fangs, the surface tension on the drop of venom 'pushes' the main body of venom into the groove on the tooth. The top of the liquid itself forms a kind of third wall.

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There's a problem here. If the venom is so tough that it forms a 'third wall' all on its own, it shouldn't go into the prey animal's tissue at all. It should just sit there in the groove, held together by its on cohesion. The venom has a fix for that. It is one of many deliciously-named thixotropic liquids. Ketchup is another. The running joke about how ketchup stays stuck in the bottle until a certain amount of shaking makes it flow so fast it floods the top of a burger has its foundation in fact. Thixotropic liquids behave like gels or foams, hanging loosely together, until a sideways force is applied. Sometimes it's rhythmic pounding on the side of a bottle. Sometimes it's fast vibrations. Sometimes it's the movement of prey, or the natural absorbtion of the prey's muscle tissue. When a sideways, or vibrational, force is applied to a thixotropic liquid, it flows fast. So the snake's venom holds together until it gets into the prey, and then gushes into the surrounding tissue.